It is known that a reverse bias voltage is applied to a transfer roller prior to applying a forward bias voltage. This prevents leak current from a photosensitive drum flowing to a forward bias applying device that generates the forward bias voltage, and can avoid a condition where start-up of the forward bias applying device is disabled under influence of the leak current.
However, in this art, an output of the reverse bias voltage is decreased after being applied. Therefore, in a case where the reverse bias voltage is set lower, start-up of the forward bias applying device could be disabled and cause transfer error and inferior print quality. On the other hand, in a case of applying a highly set reverse bias voltage while applying the forward bias voltage, it is necessary to continue applying a higher forward bias voltage, and thus, this configuration cannot be efficient.